Method for making tri-sodium phosphate



0a. so, 1928. 1,689,547. V v I J. .N. CAROTHERS G TRI'-SODIUM PHOSPHATE.

mm'mon FOR MAKIN Filed Aug. 8, 1925 J. QRROTHER":

Patented Oct. 30, 1928.

JOHN R. CABOTHEBS, F ANNISTON,

COMPANY, OF BIRMINGHAM,

ALABAMA, ASSIGLIOB TO FEDERAL PHOSPHOBUS ALABAMA, A COBIOP'ATION OF ALABAMA.

METHOD FOR MAKING TERI-SODIUM PHOSPHATE.

Application filed August a, 1925. Serial m3. mass.

This invention relates to a method for making tri-sodium-phosphate, especially adapted to the use of concentrated phosphoric acid. The object requires less e uipment and is therefore less costly, and w ich at the same time, has greater efliciency than existing methods which are adapted to the use, of dilute phosphoric acid.

With these and other objects in view, the invention consists in the novel details of a method embodying features of construcw tion and combination of equipment constituting my invention, more fully disclosed and particularly set forth in the appended claims.

Referring to the accompanying drawing, forming part of this speci cation, the appas ratus comprises an acid tank 1 containin strong phosphoric acid, which is neutralized by the addition of sodium carbonate to disodium phosphate solution in mixing tank 2, from which the solution passes through filter 3 into the receiving and mixing tank 4, where caustic soda solution is added from dissolving tank 5.' After thoroughly mix-' ing the resultant tri-sodium phosphate solution in tank 4 it is assed through filter 6 and into receiving ta ,7, from which it flows through crystallizers 8 which may have artificial coolers 8 if desired. From the crystallizers the crystals and a portion of the mother liquor pass into the centrifuge 9 where the crystals are de-watered. The mother liquor passes from the centrifuge 9 into the sump tank 10 and thence is returned by pump 11 to mixing tank 2 or'4, according to operation requirements, while the crystals pass 4 first through dryer 12" and then through cooler 13, after which they may be shipped as dried or screened crystals in any well known manner. The elements of the foregoing apparatus are conveniently illustrated as they constitute severally standard equipment. 1

In order that the operation of my method carried out in my apparatus may be clearly understood and practiced by those skilled in the art, I will outline the procedure involved. In the production of tri-sodium phosphate a large percentage of the compound is water which is present as water of crystal lization. The compound is represented is td provide a method which by the chemical formula Na,PO .12H,O, from which an inspection shows there is present approximatel lization. I proceed by adding sodium carbonate and strong phosphoric acid simultaneously in a mixingtank, where di-sodium phosphate is formed. By strong phosphoric acid I mean an acid of such concentration that it will not be necessary to employ any evaporation step in my process, but the water with the phosphoricacid, the di-sodium phosphate filter press water, and

that necessary to dissolve the caustic soda" will provide enough to supply the water of crystallization in the product. This acid may range from 50% to H PO dependiInIg on operatingconditions, and while 50% ,PO, may be continuously used provided the di-sodiuin filter press water be reduced in proportion to the additional water added by such acid, it is preferable to employ acid of higher concentrations. The phosphoric acid and sodium carbonate are added until di-sodium phosphate is formed, having, in case iron tanks are used, a slight excess of sodium carbonate. Where acid resistant tanks are used no excess of sodium carbonate is needed. The chemical reaction is represented by the following equation:

The solution is heated approximately 100 C. by a heating coil 14 to expel all dissolved carbonic acid (00,) and to coagulate the precipitate of iron, aluminum, calcium or magnesium phosphates, which will form if these impurities be present. In case this solution is higher than 1.30-1.36 specific gravity at 0., wash water or mother liquor should be added to reduce the specific gravity tank 2 the di-sodium phosphate solution passes through a filter of any standard type to remove the precipitate from the solution. The clear solution of di-sodium phosphate is then mixed with a caustic soda solution to form tri-sodium phosphate. This reaction is represented by the following equation:

NaJIPO, 121 1 0 +NaOH= W Na PO,,.12H O+H,O

to that mentioned. From the mixing 57% water of crystal- I Caustic soda solution may be prepared by dissolving solid caustic, or commercial caus of solid caustic dissolved in the commercial solution may be' satisfactorily used. The

" tals in the centrifu to mixing tank 2 or ation require.

an e at by dividin the reading that they may be tri-sodium phos hate solution is then filtered in any standar filter to remove solid matter introduced b the caustic, as well as an precipitate whic 'ay result from the addition of caustic so a to the di-sodium phosphate solution. In the addition of caustic soda, sufilcient is added until titration with half normal acid of a sample of the solution being causticized, shows a relation of the end point of phenolphthalein and the end points of phenolphthalein and methyl orvarying between 0.53 and 0.56, arrived from the phenolphthalein en point, by t e sum of the phenolphthalein and methyl orange end points. This relation of titrations is a simple and quick means of controlling the addition of caustic. After causticizing, the solution.

should have a specific gravity from 1.36'to 1.40 at 85 C. for best operation. After filtration, the solution is run into any suitable crystallizing uipment. The use of a continuous. 'crysta izer is referred, as thereb a continuous feed and 'scharge is obtaine It is desirable to add a certain percentage of mother liquor, from revious crystallizations, to aid in suspen in the crystals and to facilitate han dling. .lhe quantity of mother liquor added Wlll vary depending on operating conditions at the time, .and the only means of judging is tals produced are of the size desired, and hand e readily in the crystallizer. From the crystallizer, the crystals pass into a centrifuge where they are d'e-watered before passmg to the dryer. v ince the I melting point of tri-sodium phos hate is very' ow, it is necessary to avoi overheating and either melting or partly de-hydrating the crystals. In order that the dried crystals may be .in the best physical condition, it is preferable that they passed through a cooler aftenthe dryer,

at atmospheric temperature when stored or packed.

The solution, elimlnated from the cryse, flows to a sump tank e returned by pump 11,

from w ere it may 4 as conditions of pper- By the use of strong phosphoric acid and the apparatus as described, I am enabled to roduce .tri-sodium phosphate more efciently and less expensively than otherwise, "smce the expensive evaporation step employed in common practice is eliminated.

the fact that crys- Having thus described my invention, what claim as new and desire to secure by Letters Patent, is

l. The hereindescribed production of tri-sodium p osphate, whlch consists in subjecting sodium carbonate to the action of phosphoric acid having more than 50% H PO to form di-sodium phosphate, then adding suflicient caustic soda to convert said di -sodium phosphate into trisodium phosphate, substantially as described. s I

2. The hereindescribed process for the production of tri-sodium' phosphate, which consists in subjecting sodium carbonate to the action of stron phosphoric acid having rocess for the of tri-sodium phosphate form, substantially as described.

4. The hereindescribed rocess for the production of tri-sodium osphate, which consists in subjecting a sodium carbonate sophosphoric lution to the action of stron acid to ,form a di-sodium phospiate solution of approximately 1.33 specific gravity at (1., t en'removing all so id matter, then addinfi caustic soda solution to convert said diso ium phosphate into tri-sodium phosphate, then cooling said tri-s'odium phosphate solution until crystals of tri-sodium phosphate form, thenseparatin said crystals from the accompanying sfiution', substantially as described.

5. The hereindescribed production of tri-sodium osphate, which consists in subjecting SOdluIII carbonate to the action of stron phosphoric acid to form a di-sodium phosg ate solution of approxi- I mately 1.33 speci gravity at 85 0., then removing all solid matter therefrom, then adding caustic soda solution to convert said di-sodium phosphate into tri-sodium phosphate in a solution of ap roximately 1.38 specific gravity at 85 C., t en cooling said tri-sodium phosphate solution until crystals of tri-sodium (phosphate are formed, then separating 1sai "crfistals from the accompanying so ution, t on d in said 0 stals substantially as described g ry 6. The hereindescribed continuous .process of producing tri-sodium phosphate crysrocess for the meme-7 3 tals which consists in producing a. triand returning the mother liquor to the trisodium phosphate solution from strong acid, sodium phosphate solution, substantially as continuously feeding" such tri-sodium phosdescribed.

phete solution having an approximate spe- In testimony whereof I alfix my signacifio gravity of 1.36 to 1.410 at 85 G. to a ture.

crystallizer, and continuously Withdrawing crystals of tri-sodium phosphate therefrom, JOHN N. CAROTHERS. 

